Rotary machines, such as steam and gas turbines, used for power generation and mechanical drive applications are generally large machines consisting of multiple turbine stages. High pressure fluid flowing through the turbine stages must pass through a series of stationary and rotating components, and seals between the stationary and rotating components are used to control leakage. The efficiency of the turbine is directly dependent on the ability of the seals to prevent leakage, e.g., between the rotor and stator.
Turbine designs are conventionally classified as either an impulse (with the majority of the pressure drop occurring across fixed nozzles) or a reaction (with the pressure drop more evenly distributed between the rotating and stationary vanes) type. Both designs employ sharp, rigid teeth, known as labyrinth seals to control leakage. Traditionally, labyrinth seals of either a hi-lo (alternating teeth height) or straight shaft design are used. Such seals are employed at virtually all turbine locations where leakage between rotating and stationary components must be controlled. These include interstage shaft seals, rotor end seals, and bucket (or blade) tip seals. While labyrinth seals have proved to be quite reliable, their performance degrades over time as a result of transient events in which the stationary and rotating components interfere, rubbing the labyrinth teeth into a "mushroom" profile and opening the seal clearance.
Another type of seal used in many environments, including rotary machines, is a brush seal. Brush seals are generally less prone to leakage than labyrinth seals. A brush seal can also accommodate relative radial movement between fixed and rotational components, for example, between a rotor and a stator, because of the flexure of the seal bristles. Brush seals also generally conform better to surface non-uniformities. The result of using brush seals is better sustained rotary machine performance than is generally possible with labyrinth seals.
The combining of brush seals with labyrinth seals for turbine applications is disclosed in commonly owned pending application Ser. Nos. 08/672,665 filed Jun. 28, 1996 and 08/719,667 filed Sep. 25, 1996. The subject matter of these applications is incorporated herein by reference.
Brush seals disclosed in the '665 and '667 applications comprise bristle packs secured between a pair of rigid plates, with the bristles welded to the plates. The plates are precisely machined to fit within a stationary annular groove (the brush seals typically include four segments which, when installed, create a 360.degree. seal). As such, individual brush seals are designed for one particular diameter groove. In the field of industrial/turbo machinery, however, several different diameter seals may be required which nevertheless perform substantially identical functions. Accordingly, different tooling is required for each small diameter change, significantly increasing the cost of such seals.